Permanent magnet synchronous motors have been widely used in various occasions due to their high efficiency and high torque density. Meanwhile, permanent magnet synchronous motors utilize magnetic material with high magnetic energy, instead of traditional excitation winding. This not only avoids the negative effects resulting from traditional excitation winding, but also simplifies the mechanical structure of the motor, which improves the reliability of the motor and reduces the mechanical loss.
Some demanding applications need smooth output torque and high operating stability, such as an electric steering system and servo motor. That is, torque ripple of the motor should be maintained as small as possible, so as to achieve a smooth and accurate thrust drive. However, due to the concavity and convexity of motor structure and the coupling effect of magnetic field, permanent magnet synchronous motors suffer from relatively large torque ripple, which limits the application of these motors. Therefore, it is greatly significant to study the torque ripple suppression strategy for these motors, thus improving the smoothness of the torque.
In order to reduce the torque ripple, various methods have been proposed. Generally, these methods can be classified into three main strategies: involving stator slots and teeth, windings and rotor magnets. First, skewing is widely used to reduce torque ripple, in which the stator slots or rotor poles are skewed to reduce the cogging torque. However, the skewed stator or rotor is harder to build as for manufacturing, which also increases the cost of production. Then, the auxiliary slots or teeth are used to replace the skewing to avoid the disadvantages from skewing at the loss of efficiency. Moreover, optimization of the slot or slot-opening is also used to reduce torque ripple. Second, due to the influence between the stator winding and the cogging torque, the stator magnetic modification has been proposed to minimize the ripple, such as fractional-slot pitch windings. However, the odd and even magnetomotive force harmonics are incorporated in those windings. This means that the improper selection of the fractional slot can lead to the vibration of the stator core. Also, these methods pay much attention to reduction of torque ripple but ignore loss of output torque. Therefore, how to maintain torque density and minimize torque ripple at the same time is one key research direction.
In addition, optimization of magnets has been developed as one of the effective methods, such as reshaping magnets, using different magnet widths and asymmetry magnets. These studies result in asymmetric magnets or change the distribution of magnets by poles shifting or other methods. However, these methods only consider the reduction for cogging torque while ignoring the effects of reluctance torque on torque smoothness in inset and interior permanent magnet synchronous motors. Sometimes in inset or interior motors, the cogging torque occupies a very small proportion in the total torque ripple because of the existence of the reluctance torque. The total torque ripple was not always reduced effectively with an acceptable torque loss by the conventional magnet shifting. Therefore, how to reduce the main source of torque ripple quickly and effectively is another key research direction.